The present invention is directed to an electric assist steering system and is particularly directed to a method and apparatus for selecting steering feel and synchronizing operation between a main controller and an overseer controller for diagnostic purposes.
Electric assist steering systems are known in the art. Such systems typically include a steering wheel connected to steerable wheels of the vehicle through appropriate steering linkage, such as through a rack and pinion steering gear. An applied steering torque sensor senses applied steering torque and its output is connected to a controller. An electric assist motor is steerably connected to the steerable wheels. The controller controls energization of the electric assist motor in response to the applied steering torque. Other sensors are also connected to the controller. Such other sensors include a vehicle speed sensor. It is known to control the electric assist motor as a function of the vehicle speed. Speed proportional steering control is known in the art. As vehicle speed increases, steering assist decreases.
Diagnostic arrangements for electric assist steering systems are also known in the art. When an error is sensed in the steering system, the electric assist motor is disabled and manual steering is provided through the mechanical interconnection between the steering wheel and the steerable wheels.
In U.S. Pat. No. 3,983,953, an electric motor is coupled to the input steering shaft and energized in response to the torque applied to the steering wheel by the vehicle operator. The steering system includes a torque sensor and a vehicle speed sensor. A computer receives the output signals provided by both the torque and speed sensors. The computer controls the amount of steering assist provided by the motor dependent upon both the applied steering torque and the sensed vehicle speed.
U.S. Pat. No. 4,415,054 to Drutchas (now U.S. Reissue Pat. No. 32,222,), assigned to TRW Inc., utilizes a D.C. electric assist motor driven through an xe2x80x9cH-bridgexe2x80x9d arrangement. The assist motor includes a rotatable armature encircling a steering member. The steering member has a first portion with a thread convolution thereon and a second portion with straight cut rack teeth thereon. Rotation of the electric assist motor armature causes linear movement of the steering member through a ball-nut drivably connected to the thread convolution portion of the steering member. A torque sensing device is coupled to the steering column for sensing driver applied torque to the steering wheel. The torque sensing device uses a magnetic Hall-effect sensor that senses relative rotation between the input and output shafts across a torsion bar. An electronic control unit (xe2x80x9cECUxe2x80x9d) monitors the signal from the torque sensing device and controls the electric assist motor in response thereto. A vehicle speed sensor provides a signal to the ECU indicative of the vehicle speed. The ECU controls current through the electric assist motor in response to both the sensed vehicle speed and the sensed applied steering torque. The ECU decreases steering assist as vehicle speed increases. This is commonly referred to in the art as speed proportional steering.
U.S. Pat. No. 4,660,671 discloses an electric controlled steering system that is based on the Drutchas steering gear. In the arrangement shown in the ""671 patent, the D.C. motor is axially spaced from the ball-nut and is operatively connected thereto through a connection tube. The electronic controls includes a plurality of diagnostic features that monitor the operation of the steering system. If an error in the operation of the electric steering system is detected, the power assist system is disabled and steering reverts to an unassisted mode.
U.S. Pat. No. 4,794,997 to North, assigned to TRW Cam Gears Limited, discloses an electric assist steering system having an electric motor operatively connected to the rack through a ball nut. A vehicle speed sensor and an applied steering torque sensor are operatively connected to an ECU. The ECU controls electric current through the motor as a function of both applied steering torque and sensed vehicle speed. The current is controlled by controlling the pulse-width-modulated (xe2x80x9cPWMxe2x80x9d) signal applied to the motor. As the PWM increases, power assist increases. The ECU or computer is preprogrammed with discrete control curves that provide steering assist values (PWM values), also referred to as torque-out values, as a function of applied steering torque, also referred to as torque-in values, for a plurality of predetermined discrete vehicle speed values. Each vehicle speed value has an associated torque-in vs. torque-out control curve.
U.S. Pat. No. 5,257,828 To Miller et al., discloses an electric assist steering system having yaw rate control. This system uses a variable reluctance motor to apply steering assist to the rack member. The torque demand signal is modified as a function of the steering rate feedback.
An apparatus, in accordance with one exemplary embodiment of the present invention, for controlling a steering assist system of the type having an electric assist motor steerably connected to steerable wheels of a vehicle includes a torque sensor for providing a torque signal indicative of applied steering torque. A steering selector is used to select a desirable amount of steering assist. A vehicle speed sensor is provided for sensing vehicle speed. A controller controls the electric assist motor in response to the applied steering torque, the sensed vehicle speed, and the steering selector by adding an indexed speed value to the vehicle speed signal in response the selection of the speed selector.
An apparatus in accordance with another exemplary embodiment of the invention for controlling a steering assist system having an electric assist steering motor steerably connected to vehicle steerable wheels includes a torque sensor for sensing applied steering torque, a vehicle speed sensor for sensing vehicle speed, and a main controller for controlling the electric assist motor in response to sensed applied steering torque and sensed vehicle speed. The main controller includes dynamically varying, vehicle speed dependant tunable parameters. An overseer controller monitors operation of the main controller. The overseer includes synchronizing means for synchronizing the operation of the overseer controller and the main controller relative to the vehicle speed so that the overseer accounts for the dynamically varying, vehicle speed dependant tunable parameters of the main controller.
A method in accordance with one exemplary embodiment of the present invention is provided for controlling a steering assist system of the type having an electric assist motor steerably connected to steerable wheels of a vehicle. The method comprises the steps of sensing applied steering torque and providing a torque signal indicative of applied steering torque, selecting a desirable amount of steering assist, and sensing vehicle speed and providing a vehicle speed signal indicative of the sensed vehicle speed. The method further includes the steps of controlling the electric assist motor in response to the sensed applied steering torque, the sensed vehicle speed, and the selected steering assist by adding an indexed speed value to the vehicle speed signal in response the selected steering assist.
A method in accordance with another exemplary embodiment of the present invention for controlling a steering assist system having an electric assist steering motor steerably connected to vehicle steerable wheels includes the steps of sensing applied steering torque, sensing vehicle speed, and controlling the electric assist motor with a main controller in response to sensed applied steering torque and sensed vehicle speed. The controller includes dynamically varying, vehicle speed dependant tunable parameters. The method further includes the step of monitoring operation of the main controller with an overseer controller, the overseer controller synchronizing the operation of the overseer controller with the main controller relative to the sensed vehicle speed so that the overseer controller accounts for the dynamically varying, vehicle speed dependant tunable parameters of the main controller.